Hydraulic pumps are used in many applications. Often, they are driven from a power take off shaft, or pto, as shown in FIG. 1, forming part of the drive train of a vehicle, such as a truck. One application is a semi-trailer dump truck where the bed of the trailer is tilted upward to dump the load by a hydraulic cylinder powered by a hydraulic pump mounted to a pto on the tractor portion of the vehicle. Another application is a dump truck with the bed integral with the truck.
In normal operation, the operator would engage the pto shaft with the transmission of the tractor to begin rotation of the pto shaft. The hydraulic pump is secured to the pto shaft such that rotation of the pto shaft operates the pump to generate pressurized hydraulic fluid. A hydraulic valve is mounted to the hydraulic pump and controls the flow of the pressurized hydraulic fluid. Normally the valve will have three positions, one position to supply the pressurized hydraulic fluid to the hydraulic cylinder to lift the bed of the trailer, a second position to allow hydraulic fluid to drain from the hydraulic cylinder to lower the bed and a third, neutral position where the pressurized hydraulic fluid simply circulates around in the hydraulic pump. The position of the hydraulic valve is usually controlled by the operator with a handle on the valve.
Significant forces are created in this operation. For example, a typical lift cylinder may exert 100,000 pounds of force to lift the bed of the trailer. Ideally, the hydraulic pump is never operated unless the bed is being lifted or lowered. However, unintended operation can occur, giving raise to the potential for damage. For example, a hydraulic pump driven too long in the neutral position can overheat and fail. A cab mounted light has been used within the tractor to warn the driver the pto is engaged. However, such lights can burn out and the wiring supplying the light can be broken. A need exists for a more reliable and more effective procedure to warn an operator or driver that the hydraulic pump has been activated.